Background Our goal was to provide measurements of the posterior cerebral circulation using non-invasive imaging and advanced software analysis tools.
Methods 100 consecutive patients aged ≥40 years (50 men and 50 women) who had undergone CT angiography (CTA) but had no vascular abnormality were analyzed. Specific software was used to make vessel measurements along the center line. The length of the intracranial vertebral artery (VA), the basilar artery (BA) and the distance from the mid-basilar artery to the posterior cerebral artery (mBA-PCA) was recorded. Vessel diameter was measured at the proximal and distal ends of these vessel lengths. Vessel taper was calculated as the change in diameter in millimeters per centimeter of length.
Results The mean lengths of the intracranial VA, the BA and the mBA-PCA were 40±10.6 mm, 27.3±5.7 mm and 25.6±4.3 mm, respectively. The proximal and distal diameters were 3.9±0.8 mm and 2.8±0.6 mm for the VA and 3.6±0.6 mm and 3.1±0.5 mm for the BA, respectively. The mean mid-BA diameter and the proximal PCA diameter were 3.2±0.5 mm and 2.2±0.4 mm, respectively. There was a significant increase in arterial caliber in patients aged ≥60 years compared with those aged 40–59 years. Men also tended to have longer vessels with a larger diameter than women.
Conclusions Advanced software and non-invasive imaging can be used to perform accurate vessel analysis. The posterior circulation measurements showed an increase in arterial caliber with age. This baseline information may be useful in planning neurovascular procedures and endovascular device development.
- CT Angiography
- Posterior fossa
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The frontier of neuroendovascular therapy has expanded. Previously inaccessible vascular pathologies can now be reached due to technological developments and novel devices. As we probe further into the brain, a review of the baseline anatomic characteristics of the neurovasculature is important. These have been described in the past based on cadaveric studies1 ,2 as well as catheter angiography. The current analysis, however, was performed using advanced non-invasive imaging tools as well as an understanding of the modern-day neurovascular procedures. The methodology is based on our previously reported analysis of the anterior circulation, with this constituting the companion paper analyzing the posterior cerebrovascular circulation.
Materials and methods
This retrospective analysis was conducted under an approved institutional review board protocol.
Sample size and patient selection
The sample size calculation was based on our previously published methodology3 to obtain a representative sample with an equal number of men and women. The age cut-off was set at ≥40 years as most cerebrovascular interventions are performed in this age group. The standard deviation for cerebral blood vessels was based on the literature.1 ,2 ,4 ,5 We used a conservative standard deviation of 0.5 mm. A 95% CI with a margin of error of 0.2 yielded a sample size of 25 patients. To further accommodate any variations in vascular geometry, we extended this to 50 men and 50 women (total of 100 patients). This allowed us adequately to compare any gender differences and also any variability based on age subgroups.
The sample of 100 subjects was selected from a patient population that had undergone cerebrovascular CT angiography (CTA) either from an outpatient clinic or the emergency room. As previously,3 patients with significant vascular disease or suboptimal technique were excluded. Two experienced neuroradiologists who reviewed consecutive CTA studies spread over a 4-month period performed the imaging evaluation. Thus, the final sample comprised 50 women and 50 men aged ≥40 years.
All studies were performed on an Aquilion-64 CT scanner (Toshiba America Medical Systems, Tustin, California, USA). The technique specifies injection of 50 ml intravenous Optiray 350 (Covidien, Hazelwood, Missouri, USA) through a large-bore antecubital venous access at a rate of 3–4 ml/s followed by 50 ml saline flush. After obtaining an initial localizer at the C2–C3 level, the scan is manually initiated once the carotid arteries are opacified. Scanning is then performed with a 240 mm field of view, 512×512 matrix and 1.0×1.0 image thickness and reconstruction interval. The exposure is set at 120 kV with modulated mA. The modulated mA is an automatic feature of the scanner used that adjusts the mA based on the body habitus and body part, thus keeping the exposure to the minimum required. Multiplanar reconstruction is performed on a Vitrea workstation (Vital Images, Minnetonka, Minnesota, USA).
Target vessel measurements
An experienced neuroradiologist and a senior level radiology resident performed the image processing and analysis. The objective was to obtain measurements pertaining to vessel length and caliber from the intracranial vertebral artery (VA) segment to the posterior cerebral arteries (PCAs) while accounting for the variation at the circle of Willis. The following measurements were obtained:
The length of the dominant VA from its immediate intradural segment to the vertebrobasilar junction (VBJ).
The diameter of the intracranial VA at its immediate intradural entry and the diameter just proximal to the VBJ.
The length of the basilar artery (BA) from the VBJ to its terminus.
The diameter of the BA at its origin and terminus.
The length of the BA from its mid-point to the P1–P2 junction of the dominant PCA.
The diameter of the BA at its mid-point and the diameter of the PCA at the P1–P2 junction.
The vessel taper and tortuosity index (TI) were calculated for the VA, BA and the mid-BA to P1–P2 junction based on the diameter at the proximal and distal ends of these vessel lengths, as subsequently described.
Image processing and vessel analysis
Image processing was performed after importing the source CTA data into a dedicated workstation running the Vitrea Core Software V.6.2.1 (Vital Images). The vascular package of the software was used to create a three-dimensional model of the posterior circulation with automated removal of bone and soft tissues. After segmenting out the vessel of interest, a center line was obtained along its long axis additionally generating a curved planar reformation and cross-sectional model of the target vessel. The model also generates multiplanar reformats that can be correlated with the three-dimensional model. The cross-sectional area of the vessel perpendicular to its long axis was measured at the proximal and distal ends of the target vessel and the diameter calculated based on πr2. The vessel length, TI and taper were then calculated between these proximal and distal points. The TI represents a ratio of the curved length of the blood vessel to the straight line distance between two points, a higher TI indicating a more tortuous blood vessel. The curved length is the true length of the blood vessel along its center line. Thus, a higher TI will indicate a more tortuous vessel. For example, a curved length of 4 cm and a straight line distance of 2 cm between two points gives a TI of 2, or the curved length is twice as long as it would be if the vessel was straight.3 The vessel taper represents the decrease in caliber per unit length and can be obtained by dividing the difference between the proximal and distal diameters over the length of the vessel segment, giving a taper/mm of vessel length.
Prior to the actual measurements, the methodology was applied to ‘practice’ cases, specifically to obtain consistency in defining the intradural vertebral segment—the start point for the measurements. The intradural segment was determined on the axial bone-subtracted model where the VA could be observed at the dural ring by a slight change in caliber. The two interpreters then independently obtained these measurements for the entire sample. The variation in the PCAs regarding the presence of unilateral or bilateral fetal origins was also recorded.
The data were imported into the JMP statistical software V.9 (SAS Institute, Cary, North Carolina, USA). The data from the two interpreters were correlated and the mean of the two constituted the unit of analysis. Mean values and standard deviations for all the measurements were generated along with the respective 95% CIs. We performed a bivariate analysis of age with the vessel parameters to determine any association of vessel size with age and used the Student t test to compare the means based on gender and side. The mean value of the two readers was evaluated for correlation and formed the units of analysis.
The mean age of the sample was 57.4±13.6 years with mean ages for men (n=50) and women (n=50) of 56.1±13.0 years and 58.7±14.1 years, respectively (p=0.3).
There was a strong positive correlation between the two interpreters: VA length (R2=0.9), proximal VA diameter (R2=0.6), distal VA diameter (R2=0.8), BA length (R2=0.8), proximal BA diameter (R2=0.8), mid-BA diameter (R2=0.7), distal BA diameter (R2=0.8) and PCA diameter at the P1–P2 junction (R2=0.5).
The vascular length, the diameter at the proximal and distal ends, the vessel taper per cm and the TI for the three vertebrobasilar segments are shown in figure 1. There was no significant variation in either the length or the caliber of the arteries based on gender; however, in patients aged ≥60 years the VA and BA had larger diameters than in those aged 40–59 years (table 1). There was no significant difference in the length of the vessels between the two age groups.
Most of the patients (n=74, 74%) had symmetrical or almost symmetrical bilateral PCAs. A unilateral fetal origin of the PCA was seen in 20 patients (20%) while PCAs with a bilateral fetal origin were seen in six (6%). In patients with bilateral fetal origin PCAs, the diameters of both the VA and BA were significantly smaller than in those with two symmetrical PCAs. The impact of a single fetal origin PCA was less significant, especially on the diameter of the VA (table 2).
The vessel taper, indicating the change in arterial caliber over 1 cm for each vessel segment, as well as the TI is shown in figure 1. There was no difference in these values based on gender or age.
Advances in high-resolution non-invasive neurovascular imaging and development of robust post-processing software has allowed for an easier and sophisticated vascular analysis. The almost ubiquitous availability of multidetector CT scanners makes them an ideal tool for rapid vascular imaging. The image quality of current CTA is approaching conventional digital subtraction angiography (DSA),6–8 especially with the use of bone-subtraction algorithms allowing for better visualization at the skull base,9 ,10 and measurements of the anterior cerebral circulation using CTA3 have closely matched older data relying on DSA.5 Our current analysis uses an automated cerebrovascular software package which allows precise measurements across a prescribed vascular trajectory yielding both basic information such as length and caliber and also tortuosity, which is an important consideration in cerebrovascular interventions.
While data regarding vascular geometry in the anterior circulation are available,1–5 similar measurements covering the posterior cerebral circulation are lacking. There have been descriptions of anatomic variations at the circle of Willis,11 ,12 especially the effect of fetal type configuration on cerebral collateral circulation.13 We demonstrate that the presence of a unilateral or bilateral fetal origin PCA significantly impacts the caliber of the intracranial vertebrobasilar circulation (table 2), and such information may be useful when planning an intervention.
Our data show the effect of age on vessel caliber, with subjects aged ≥60 years having significantly larger diameter than those aged 40–60 years (table 1). This finding is similar to previous reports,3 ,5 ,14 and changes in pulse pressure and arterial compliance may be contributing factors to this increase in caliber.15 ,16 Additionally, in contrast to a previous report regarding the anterior circulation,3 the current study shows that there is a distinct and, in some measurements, a significant trend for men to have longer and/or larger caliber measurements (table 1). Also, unlike the reported vessel taper of almost 0.5 mm per centimeter for the intracranial internal carotid artery,3 the taper in the posterior circulation is much more gentle (figure 1) with the greatest change in caliber seen between the mid-BA and the PCA.
This brief analysis provides geometric information for the posterior cerebral circulation using advanced imaging techniques. The baseline data can be helpful in planning neurovascular interventions and may also aid in endovascular device development.
Contributors ATR contributed to the study design, methodology and analysis. DR, JPH and EAW contributed to the data collection and methodology.
Competing interests None.
Ethics approval Ethics approval was obtained from the Institutional Review Board.
Provenance and peer review Not commissioned; externally peer reviewed.